In recent years, partly on account of the higher power levels that have become possible, there has been a remarkable growth in the use of laser beam machines which employ fiber lasers. However, the resonance state of the laser light source built into a laser beam machine is destabilized by the entry of outside light, disturbing the oscillation state. Disruption of the oscillation state is particularly severe when the light that has been generated is reflected by intermediate optics and returns to the light source. To keep this from happening, an optical isolator is generally provided just in front of the light source, for example.
Optical isolators are made of a Faraday rotator, a polarizer situated on the input side of the Faraday rotator, and an analyzer situated on the output side of the Faraday rotator. The Faraday rotator is used by applying a magnetic field parallel to the propagation direction of light, at which time a polarized component of light, whether traveling forward or backward through the Faraday rotator, rotates only in a fixed direction. In addition, the Faraday rotator is adjusted to a length such that the polarized component of light rotates exactly 45°. When the polarizer and analyzer planes of polarization are offset by 45° in the direction of rotation by forward-traveling light, polarized light traveling forward coincides with the polarizer position and with the analyzer position and thus passes through each. By contrast, polarized light traveling backward from the analyzer position rotates 45° in the opposite direction from the direction of angle offset by the polarizer plane of polarization that is offset 45°. As a result, the returning light has a plane of polarization at the polarizer position that is offset 45°−(−45°)=90° with respect to the polarizer plane of polarization, and thus cannot pass through the polarizer. Hence, the optical isolator functions by allowing forward-traveling light to pass through and exit therefrom and by blocking backward-traveling return light.
Materials hitherto know to be capable of use as the Faraday rotator in optical isolators include TGG crystals (Tb3Ga5O12) and TSAG crystals (Tb(3-x)Sc2Al3O12) (see JP-A 2011-213552 and JP-A 2002-293693 (Patent Documents 1 and 2, respectively)). TGG crystals have a relatively large Verdet constant of 40 rad/(T·m), and today are widely used in standard fiber laser systems. TSAG crystals have a Verdet constant which is reportedly about 1.3 times that of TGG crystals and is likewise a material used in fiber laser systems.
In addition, JP-A 2010-285299 (Patent Document 3) discloses a single crystal or ceramic composed primarily of the oxide (TbxR1-x)2O2, wherein 0.4≦x≦1.0 and R is selected from the group consisting of scandium, yttrium, lanthanum, europium, gadolinium, ytterbium, holmium and lutetium. Oxides composed of these constituents have Verdet constants of 0.18 min/(Oe·cm) or more, with the largest Verdet constant mentioned in the examples provided therein being 0.33 min/(Oe·cm). The same document also mentions, in the text thereof, a Verdet constant for TGG of 0.13 min/(Oe·cm). Hence, the difference between the Verdet constants for both is 2.5-fold.
An oxide composed of substantially similar components is disclosed in JP-A 2011-121837 (Patent Document 4) as well, where it is mentioned that this oxide has a larger Verdet constant than a TGG single crystal.
When, as in Patent Documents 3 and 4 above, an optical isolator having a large Verdet constant is obtained, the total length required for 45° rotation can be shortened, which is desirable in that it makes a smaller optical isolator possible.
One material that has a very large Verdet constant per unit length is iron (Fe)-containing yttrium iron garnet (YIG) single crystals (JP-A 2000-255947 (Patent Document 5)). However, iron has a large light absorption at a wavelength of 0.9 μm, which absorption affects optical isolators used in the wavelength range of 0.9 to 1.1 μm. This makes optical isolators that use such yttrium iron garnet single crystals difficult to employ in fiber laser systems where the trend is clearly toward higher power levels.